A study is made of the influence on hemagglutination of various physicochemical parameters, such as: cell shape, cell distance, extracellular colloid-osmotic pressure, degree of hydration of the cell surface, and cell ζ-potential. To a varying extent these are changed by: the addition of divers soluble polymers, the action of enzymes, the presence of various salt ions, centrifugation, agglomeration at low ionic strength, complex coacervation with cationic polymers, and the interaction with different kinds of antibodies. Among the more important parameters are cell distance and cell shape (particularly when the latter tends to spiculation), while the influence of ζ-potential appears to be of fairly minor consequence. The action of dissolved polymers is mediated through polymer bridging, increase in extracellular colloidosmotic pressure, decrease of cell surface hydration, and change in cell shape. A peculiarity of dextran is that in addition to all of the above, it causes pronounced erythrocytic spiculation.

Scanning electron micrography of red cells agglutinated by various blood group antibodies has shown that the agglutinates formed are of two completely different morphologic types. Type I, obtained with anti-A sera (IgM and IgG) as well as with anti-A(1) lectin, consists of interlaced, crenated and spiculated cells, while type II, obtained with anti-D sera (complete and incomplete) consists of densely but randomly packed smooth, discoid cells. Papain treatment results in indented, spheroid cells with a much corroded surface. Coombs-type antiglobulin agglutinates consist of spheroid cells, the coating of which tends to form a continuous envelope, interconnecting several cells. The influence of heterogeneic and of viral agglutinins is also illustrated.